• null is a value that means a variable doesn't hold a reference to an object. The variable exists, but it doesn't refer to anything

• A nullable type can hold null values in addition to its base type. You define a type as nullable by adding a ? to the end of it.

• To access a nullable variable's properties and functions, you must first check that it's not null

• if the compiler can't guarantee that a variable is not null in between a null check and its usage, you must access properties and functions using the safe call operator ?.

• You can chain safe calls together.

• To execute code if (and only if) a value is not null, use ?. let

• the Elvis (?:) is a safe alternative to an if expression

• The not-null assertion operator (!!) throws a NullPointerException if the subject of your assertion is null.

• An exception is a warning that occurs in exceptional situations. It's an object of type Exception

• Use throw to throw an exception

• Catch an exception using try/catch/finally

• try and throw are expressions

• Use a safe cast as? to avoid getting a ClassCastException